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two-dimensional conformal field theory correlation functions on Riemannian sur-faces of higher genus g. The expansion parameter is g --) with gQ the stringcoupling and depends on the dilaton o via gQ = exp(c). Suppose we compact-ify the ten-dimensional superstring on a six-dimensional background such thatN = 1 supersymmetry is preserved in four dimensions. One can then compute aneffective four-dimensional supergravity action for the massless modes. The non-renormalization theorems for holomorphic couplings generalize naturally to thestring case. For the holomorphic couplings W and f we expect that beyond tree-and one-loop level, respectively, non-perturbative corrections are present. Sincewe still lack a complete second quantized version of string theory, one must arguefor the existence of these non-perturbative corrections by the analogy with fieldtheory. Stringent tests of their presence in decoupling limits, or in cases whereduality maps such effects to classical effects, provide overwhelming evidence thatthis analogy is correct.From the early days of the heterotic string, effects that are non-perturbativefrom the worldsheet perspective have been a field of active interest. Such config-urations arise as Euclidean closed strings wrapping topologially non-trivial two-cycles of the compactification manifold [5, 6]. Being localized in four dimensions,they are called worldsheet instantons, in analogy to the Euclidean topologicallynon-trivial solutions of Yang-Mills theory. Their contribution to the couplings isnon-perturbative in the worldsheet expansion parameter a', but not in the stringcoupling g .However, the past one and a half decades have witnessed major progress inthe understanding of objects in string theory which are non-perturbative alsofrom the spacetime point of view. It has been shown that p-brane solutions ofthe supergravity equations of motions are truly non-perturbative objects in stringtheory. In particular for the large class of D-branes, the quantum theory aroundthe classical solution is known to be given by an open string theory with end-points on the D-brane [7]. These D-branes carry charge under certain Ramond-Ramond p-forms and also have tension scaling like T, = g 1. Such objectsare indeed present in four-dimensional Type II string vacua preserving N = 1space-time supersymmetry in two different ways. Firstly, D-branes can fill four-dimensional space-time and wrap certain cycles of the internal manifold. TheseD-branes carry both gauge fields and chiral matter fields which are observed asphysical fields on the four-dimensional effective theory localized on the D-brane.The past years have seen many attempts to realize realistic gauge and matterspectra on such intersecting D-brane models. This includes investigations of D-branes on compact manifolds as well as on non-compact geometries. Beyond themere construction of models, a formalism has been developed to compute theresulting N = 1 supersymmetric four-dimensional effective supergravity action.Soon it was realized that D-branes play an important role not only as thehosts of this effective field theory, but also as actors in it: Euclidean D-braneswrapping entirely a topologically non-trivial cycle of the internal manifold appear